A girl named Veronica could easily be called Nickey as a nickname, so if you name her Veronica, she could go by Nickey a lot of the time, but she will still have an alternative to use when she's older. Something does look a bit off with Nickey Nikole but Veronica is beautiful and they would still be Natasha and Nickey at home

I agree that you should give your next girl her own middle name, but if you do decide to go with Nikole, use that spelling at least.

How about:
Veronica Nova
Veronica Novalee
Veronica Nina
Veronica Naomi
Veronica Nora
Veronica Nancy
Veronica Norelle

or continue the double letter trend and go:
Veronica Velma
Veronica Victoria
Veronica Vera
Veronica Vesper
Veronica Verity
Veronica Vanya

Veronica Marie

While those are beautiful names, I have to say that "Nickey Nikole" does sound a little funny. I like "Veronica Nikole". That way you have a liitle of both

your daughters are gonna want there own Identity's they are gonna think its bad enough that they may have to share many things especially when they get older and the last thing they are gonna want is to share the same middle name spelling or not! I like Veronica besides if you do different name patterns for the two it will be different and more unique and creative most of all!